The present invention relates to shielding undesirable background electromagnetic radiation from radiation detectors. Such shielding is particularly applicable to infrared detectors and systems. However, shields in accordance with the present invention are equally applicable to other detector systems in which it is desirable to shield background radiation in the spectral or electromagnetic energy band of the system detectors.
With background limited infrared detectors, individual detector sensitivity or figure of merit is generally recognized to be inversely proportional to the square root of background radiation. Therefore, it is desirable to limit detector background so that each detector in an array sees only the applicable optics of the infrared system.
Lowering background radiation by shielding at the optics is cumbersome and adds weight or complexity to the system. Accordingly, shielding at the detector has long been recognized as an effective and workable approach to limiting background radiation. In the past, individual detector elements have been shielded by a variety of means. In the instance of a linear array of detectors, a slotted type of shield has often been used. In this configuration, parallel strips of metal or other reflective material have been mounted on the detector substrate or in the dewar parallel to the detector array in order to limit a certain amount of stray energy from striking the detector from sources other than the collection optics of the system. These systems have the disadvantage that they only reject stray energy in one direction, perpendicular to the linear array. In addition, if the mask is placed anywhere other than on the detector substrate, stray energy may be reflected off internal surfaces of the system onto the detector array, further reducing the efficiency of the shield.
In one prior art cold shield design, described in U.S. Pat. No. 3,963,926, S. R. Borrello, "Detector Cold Shield" " (hereinafter Borrello) bulk silicon is adhered to a detector substrate, and individual holes corresponding to each detector element are created in the material by an etching process. The walls of the holes and the three-dimensional bulk of the material mask serve to help restrict the field of view of each detector to the cone of energy focused by the collection optics.
With the Borrello design, the sides of the holes must necessarily extend far above the detector surface through the mask material bulk, which is necessary to provide mechanical support for the array mask or cold shield structure. In a low f-number optical system, where the cone of energy focused by the collection optics is relatively broad, it may be impossible to properly shield the detectors while providing the necessary mechanical rigidity, since the walls of the mask material may occlude the desired optical beam if the material is too thick.
A still further disadvantage of the Borrello shield relates to the fact that, because of the directionality of preferential etching in silicon, it is impossible to match the configuration of the cold shield apertures with the configuration of conventional detectors, thus reducing shielding efficiency. Another disadvantage of the cold shield disclosed in Borrello is that the preferential etching of silicon may lead to a coalescing of etched holes in closely spaced detector arrays, which may mechanically weaken the structure. These disadvantages are thoroughly discussed in U.S. application Ser. No. 248,127 W. J. White, "Etchable Glass Cold Shield for Background Limited Detectors", filed March 27, 1981 (hereinafter White). The cold shield disclosed in the White application overcomes these problems and in addition provides other advantages discussed in the application.
The shield disclosed in White comprises a member of etchable glass having apertures formed therein, the position of the aperture edges being in predetermined relation to edges of detectors within an electromagnetic radiation detection system. The shield is employed to shield the detectors from electromagnetic radiation generated outside the field of view of the system optics, thereby increasing the sensitivity of the system.
Although the White shield provides many advantages over that disclosed in Borrello, the present application has additional particular advantages. The present invention allows a very thin cold shield to be placed very closely to the detectors, such as directly on or within the substrate that is supporting backside illuminated detectors. As explained further below, this permits cold shielding a very high density array of detectors.
Further, the present invention uses an opaque absorber or reflector, such as a thin film, which is much easier to pattern than trying to etch holes in a physical member.
In addition, the present invention allows a one piece structure, allowing one to get the detector array and cold shield all in one piece, so that one does not have to be mounted to the other. Such a structure provides a higher yield process.
Although it would be possible to take either the Borrello or the White cold shield and mount it to the front surface of a detector array so that back surface detectors would be shielded, such an approach, for a very high density array, would present problems involving the thickness of the shield itself, especially with the Borrello shield (this aspect is thoroughly explained in the White application). When one starts from a detector and goes out toward a source of radiation, there is a certain included angle that energy is being collected within. In a high density array, with closely spaced detectors, or even in an array of not so closely spaced detectors, the farther away from the detectors one gets, the more the included angles start to overlap. As soon as they overlap, a physical structure cannot be used for the cold shield since there would then be overlapping holes, which means there would be no material between the holes.
However, through the present invention, a cold shield can be placed very close to the detectors by, for example, in the case of back surface detectors, providing a thin film directly on to the front surface of the detector array substrate. With this approach, one can get close enough to the detectors with a sufficiently thin material so that the shield apertures do not overlap.
In contrast, if one has to make a physical thing, such as the Borrello or White cold shield, it has to have sufficient thickness to hold itself together. Thus, in a typical modern array, in attempting to use such a shield, one would, in some applications, end up with no structure.
Through the present invention, a cold shield is provided for shielding individual elements of an infrared detector array of closely spaced detector elements, while providing good mechanical rigidity. While the present invention has particular advantages for low f-number optical systems or high density arrays, it is not limited to such systems. Further, the present invention provides, through standard semiconductor processing technology, a means of optimally placing a cold shield mask in predetermined relation to the individual detectors, while eliminating the problem of aligning separate shields and arrays, particularly for backside illuminated detector arrays. Also, as with the White cold shield, cold shield apertures provided by the present invention may be matched in configuration to the configuration of conventional detectors. In addition, the present invention provides a cold shield which is easily and economically manufactured.